Recently, in the catalytic cracking of hydrocarbon oils, there is a desire to upgrade a less expensive feedstock hydrocarbon oil of lower quality, while there is an increasingly growing tendency for feedstock hydrocarbon oils to become heavier.
Heavy feedstock hydrocarbon oils contain a large amount of metals such as nickel, vanadium and the like, and the metals almost wholly deposit on the catalyst.
In particular, it is known that when vanadium deposits and accumulates on the catalyst, it destroys the crystal structure of the crystalline aluminosilicate zeolite which is an active ingredient of the catalyst and therefore a considerable decrease in catalytic activity is brought out and the amount of hydrogen and coke yielded is increased.
On the other hand, it is known that nickel causes catalytic dehydrogenation upon deposition and accumulation on the catalyst surface and therefore increases the amount of hydrogen and coke yielded and, as a result, nickel causes problems, for example, that the regeneration tower temperature is elevated.
When a feedstock hydrocarbon oil containing a large amount of a heavy bottom oil (e.g., topping residue or vacuum distillation residue) is used, not only does the influence of the metals become greater but also the sulfur compounds contained in the bottom oil cause a problem that the amount of SOx in the flue gas from a catalyst regeneration tower increases and a product oil fraction, in particular a gasoline, has an increased sulfur concentration.
Furthermore, increase of the treated amount of bottom oils leads to an increase in catalyst makeup amount and causes problems relating to increase in catalyst cost and load imposed on the environment due to an increase in the amount of waste catalysts.
Up to now, in order to deactivate poison metals such as vanadium or the like to be deposited on a catalyst to thereby improve the metal resistance of the catalyst, various techniques which incorporate a basic compound or the like as a metal deactivator into the catalyst have been proposed. Examples include a technique in which a water-soluble compound of an alkaline earth metal or the like is ion-exchanged with a zeolite or inorganic oxide matrix and a technique in which a water-insoluble oxide (e.g., dolomite, sepiolite, anion clay, or the like) is incorporated into an inorganic oxide matrix (JP-A-62-57652, JP-A-63-182031, JP-A-3-293039, etc.).
Although the compounds of alkaline earth metals have the effect of deactivating poison metals, they have no cracking ability when used alone. Consequently, they are used after having been incorporated as a metal deactivator into an inorganic oxide matrix having a cracking ability, as described above. However, in the catalyst, since the alkaline earth metal (especially a magnesium compound or the like) moves in the form of a low-melting compound during catalytic cracking reactions and the basic nature thereof destroys the crystal structure of the crystalline aluminosilicate zeolite, the thermal stability is reduced.
The catalyst described above obtained by incorporating a compound of an alkaline earth metal into a crystalline aluminosilicate zeolite through ion exchange has problems, for example, that the gasoline product obtained through catalytic cracking reactions has a reduced octane number (RON).
Furthermore, when anion clay or the like is used, the clay naturally occurring is rare and hence greatly raises the catalyst cost, while synthetic products of the clay also are not inexpensive, resulting also in an increased catalyst cost.
In addition, when a compound of an alkaline earth metal is dispersed as a metal deactivator in an inorganic oxide matrix, the pH of the catalyst slurry fluctuates considerably due to the basic nature of the compound so that it is difficult to produced the catalyst.
In particular, magnesium compounds dissolve away in the step of catalyst washing with ammonia, an aqueous ammonium sulfate solution or the like (removal of an alkali metal such as sodium or potassium from the catalyst). It is hence difficult to wash catalysts containing magnesium, and the incorporation thereof into catalysts is problematic.
On the other hand, an additional advantage of the catalyst compositions described above having the effect of trapping vanadium is that they have SOx-binding ability (see U.S. Pat. No. 4,889,615, etc.). The ability is effective in diminishing SOx in the discharge gas from a regeneration tower and reducing the sulfur content of a product oil.
Heavy hydrocarbon oils, in particular, have a high sulfur content, and the sulfur compounds deposit on the catalyst together with coke and become SOx in the regeneration tower of the FCC apparatus. SOx reacts with the basic metal oxide and is thus trapped in the catalyst. The sulfur thus trapped can be separated and recovered after it is converted to hydrogen sulfide through reactions in the riser. It is known that the catalyst compositions thus diminish SOx in the combustion gas and reduce the sulfur content in the product oil.
However, when nickel accumulates on the catalyst surface, there are often cases where the metal deactivator described above has no deactivating effect on the nickel. Accordingly, a technique of feeding a specific antimony compound (organoantimony, etc.) to a feedstock hydrocarbon oil to thereby deactivate the nickel deposited on the catalyst surface has been proposed (JP-A-63-63688, JP-A-1-213399, etc).
However, the antimony compound accumulates as a metallic antimony deposit (low-melting compound having a melting point of from 500 to 700° C.) on the control valve and the like in the FCC apparatus.